Transfer RNAs (tRNAs) are essential biological adaptor molecules connecting mRNA to protein synthesis. Given their important biological role, correct maturation and modification of tRNAs is strictly required for cell health and viability. Ths is particularly salient in mitochondria, where mitochondrial tRNA gene mutations are the prevalent cause of mitochondrial disease. In organellar genomes, tRNAs punctuate rRNA and protein coding genes, making 5' end processing of tRNA transcripts essential not only for tRNA maturation, but also for the 3' end processing of the preceding sequences. Across all domains of life, tRNA 5' end maturation is catalyzed by the essential enzyme ribonuclease P (RNase P). Until recently all known RNase P enzymes were thought to include a catalytic RNA component. However, the discovery of Protein Only RNase P (PRORP) in human mitochondria and A. thaliana chloroplasts, mitochondria, and nuclei has shifted this paradigm. Correct tRNA maturation by PRORP is essential for human health; mutations in PRORP and its substrates that disrupt tRNA 5' end processing are linked to diseases including maternally inherited essential hypertension, mitochondrial myopathy, MELAS, and HSD10- disease. Our work will use biophysical, biochemical and cell- biological techniques to determine the structure and mechanism of PRORPs, as well as develop an in vivo model system for assessing the physiological role of PRORPs. These studies will establish a framework for understanding the molecular basis of, and ultimately treating, a range of mitochondrial diseases. Together, our highly interdisciplinary work will provide the first structural and mechanistic information on mitochondrial tRNA 5' end processing in higher eukaryotes.